EP 636463 A1 has disclosed this so called in-situ foaming technique. This known technique comprises the steps of providing a sheet of a thermoplastic material comprising an amount of a suitable physical blowing agent (a swelling agent or solvent), placing this sheet between two fibre-reinforced cover layers of a similar thermoplastic material, placing the assembly of thermoplastic core and fibre-reinforced cover layers between two heated press plates, supplying heat and pressure to the assembly and upon reaching a foaming temperature causing foaming of the thermoplastic core by increasing the spacing between the press plates, cooling the press plates when a predetermined foamed core thickness is obtained, while the sandwich panel thus obtained is kept under pressure, followed by a drying step to reduce the content of remaining physical blowing agent or solvent.
NL2012710 C has disclosed an intermediate product comprising a skin and a foamable layer, further comprising a reinforcing layer, which is embedded in the foamable layer or between the foamable layer and the skin. Upon foaming this reinforcing layer is embedded in the foamed layer or between the foamed layer and the skin.
From DE 1267416 a supporting mould for manufacturing insulating bodies or containers made of rigid plastic foam is known, wherein the form parts that are facing the insulating body to be manufactured, comprise at least partially two or more layers approximately parallel to the mould surface, which layers are made of different materials. These layers are constructed such that in the supporting mould from the inside to the outside metal heat-storing layers and heat-insulating foam layers are alternately arranged.
WO 2006080833 A1 has disclosed that during the drying step at elevated temperature of the in situ foaming technique the remaining physical blowing agent is preferably removed, while the outflow thereof via the peripheral edges of the foamed core is restricted.
The in situ foaming technique can also be used for manufacturing such sandwiches starting with a core part comprising a thermoplastic material that contains a chemical blowing agent, between cover parts, as disclosed e.g. in WO 2015065175 A1 and WO 201506176 A1.
In sandwich products having a symmetrical design in the thickness direction and obtained using said in situ foaming technique the adhesion between the cover layer(s) and the foamed core is stronger than the bonding between the cells of the foam. Thus upon excess mechanical loading along the interface between core and cover layer failure primarily occurs in the foamed core.
Now it has appeared that upon using this prior art in situ foaming technique in order to manufacture sandwiches having an asymmetrical configuration (taking the core as centre) in the thickness direction the adhesion between a (fibre-reinforced) cover layer and the foamed core may fail, and that the flatwise tensile strength is low compared to a symmetrical design having a similar density of the foamed thermoplastic core. Such asymmetrical configurations may be desired for applications wherein both planar faces of a sandwich panel serve different purposes and thus require different properties.